This invention relates generally to the art of polymers and more particularly to the art of polymeric packaging films.
Polymeric films are ideally suited for packaging materials because of the ease of fabrication thereof and because of the ability of some polymeric films to maintain a packaged product in a desirable or unaffected condition. Particularly associated with the preservation of food articles is the ability of many polymeric films to resist the permeation of deleterious atmospheric gases into the contained product. One particular material which has been extremely successful for packaging food articles is a copolymer of vinylidene chloride with one or more other monomers usually vinyl chloride. This copolymer is normally referred to commercially as saran. Saran has many characteristics, however, which make it a less than totally satisfactory packaging material. The chief disadvantage of saran is the difficulty associated with the forming of film products from resin melts. Another questionable aspect of saran is the composition thereof which belongs to a broad class of organic halides which are being seriously questioned from a health and environmental standpoint.
A proposed replacement for saran as a material resistant to oxygen permeation is the hydrolyzed product of ethylene and vinyl acetate. This hydrolysis product is essentially a copolymer of ethylene and vinyl alcohol since the hydrolysis is normally greater than 96 percent complete. Copolymers of ethylene and vinyl alcohol and processes for producing them are fully described in U.S. Pat. Nos. 2,386,347 and 3,585,177 herewith incorporated by reference. A process for forming such a composition into a film is described by Chiba et al in U.S. Pat. No. 3,419,654 also herewith incorporated by reference.
Films composed of copolymers of ethylene and vinyl alcohol have been found to have a permeability to oxygen which is comparable to that of commercial saran while not possessing any halide moieties within the polymer structure.
A problem, however, with copolymers of ethylene and vinyl alcohol, while posssessing excellent low oxygen permeability characteristics, is the effect that moisture has upon the desirable properties of the copolymer. Copolymers of ethylene and vinyl alcohol are soluble to some extent in water and are affected by moisture such that while retaining dimensional integrity the resistance to oxygen transmission is almost completely diminshed. Thus any use of such copolymers for an application where low oxygen permeability is required must include measures to prevent moisture from coming into contact with the copolymer and must totally preclude the use of such copolymer in contact with a moisture bearing food article.